In humans, a common risk factor for the development of invasive breast cancer is dense breast tissue, detected by mammography. This dense tissue is associated with increased stromal collagen, increased epithelial cells, and decreased fatty tissue. Recent studies in transgenic mice indicate that increased collagen density in breast stromal tissue plays a causative role in promoting both the formation and invasiveness of breast tumors. Additional studies implicate tissue rigidity downstream of extracellular matrix (ECM) deposition and/or cross linking in promoting aggressive, invasive cellular phenotypes. Conversely, basement membrane ECM proteins that underlie normal and carcinoma in situ epithelial cells are thought to inhibit tumor progression. Because of the complexity of extracellular matrix-tumor cell interactions, it is difficult to fully isolate and understand the various effects of extracellular matrix on tumor progression using traditional biological approaches. We therefore propose to use an interdisciplinary approach in which we develop a 3-dimensional cell-based multiscale mathematical model of ECM-breast cancer interactions. Using this model, we will perform in silico experiments to test the overall hypothesis that ECM rigidity and stromal collagen fibrosis provide an environment that promotes tumor progression and invasion. We will specifically test the role of collagen fibril density, width, alignment, and crosslinking and the role of basement membrane ECM and proteases on cancer growth and invasion. All modeling will be fully integrated with experimentation to obtain realistic parameters and separately test predictions. We anticipate that this project will identify critical microenvironmental factors promoting breast cancer progression and lay the groundwork for future therapeutic intervention.